FACTBOX - Chronology of Gulf of Mexico oil spill

(Reuters) - A drilling rig explosion on April 20 left 11 workers missing and presumed dead, and the rig's subsequent collapse unleashed a major oil spill that threatens the ecosystems and economy of the U.S. Gulf of Mexico.

Also threatened is the heart of U.S. energy production, as a giant, unprecedented underwater leak spreads oil across the northern Gulf of Mexico, threatening areas from Florida to points west of the of the Mississippi River.